Function classify_generic_usage 

fn classify_generic_usage(ty: &Type, generic_name: &Ident) -> GenericUsage

Classifies how a generic type parameter appears in a type.

Strips Option<...>, Result<..., E>, and ExcludeNull<...> wrappers before inspecting the inner type. Any generic type wrapping T that isn’t Option, Result, or ExcludeNull is treated as a container. If the container doesn’t implement SqlContainerType, the generated code won’t compile (a clear error).